ok. i do have a turbo box so i do have more space than normal.
Think in terms of people walking down a narrow alley, if those in front stop, the peeps at the rear will just push them along, so if that alley opens up to the size of football pitch ! the people exiting that alley will often lose direction, this is what will happen with your 50mm tube, the flow will drop out of laminar and result in increased turbulent flow, this is bad when trying to get air into a turbo as they "thrive" on laminar flow and then direct it straight on to the impeller to increase the revs and therefore increase the boost, once the exhaust gases have done there job getting rid of it is then the job of the rest of the exhaust system and the bigger the better as any restriction will cause loss in power/torque, however Phil is right there comes a point where the pipe size is big enough and any bigger is just extra weight to carry, On your build, it depends on the boost you are aiming for, more boost more exhaust to lose.....
